Como Calcular O Peso Do Feto Pelo Ultrassom

Calculating fetal weight using ultrasound measurements is an essential part of prenatal care. This guide explains the process, provides a calculator, and offers interpretation guidance.

How to calculate fetal weight

The most common method for estimating fetal weight is using the Hadlock formula, which combines the biparietal diameter (BPD) and abdominal circumference (AC) measurements from ultrasound scans.

Note: Ultrasound measurements should always be performed by a qualified healthcare professional. This calculator provides an estimate based on standard formulas.

Step-by-step process

Schedule a prenatal ultrasound between 18-22 weeks of gestation
Measure the biparietal diameter (BPD) - the distance between the fetal skull bones
Measure the abdominal circumference (AC) - the circumference around the fetus' abdomen
Record the measurements in millimeters (mm)
Use the Hadlock formula to calculate estimated fetal weight

Recommended measurement times

Early second trimester (18-22 weeks)
Late second trimester (28-32 weeks)
Before delivery (36-40 weeks)

Fetal weight formula

The Hadlock formula is the most widely used method for estimating fetal weight:

Hadlock Formula:

Estimated Fetal Weight (grams) = (BPD × AC × AC) / 250

Where:

BPD = Biparietal diameter (mm)
AC = Abdominal circumference (mm)

This formula provides a reasonable estimate of fetal weight, though it's important to note that actual birth weight may vary by several hundred grams.

Example calculation

If a fetus has a BPD of 50mm and an AC of 220mm:

Estimated weight = (50 × 220 × 220) / 250 = 10,120 grams (10.12 kg)

Factors affecting fetal weight

Several factors can influence fetal weight estimates:

Factor	Effect on Estimate
Gestational age	Earlier estimates may be less accurate
Multiple pregnancies	May require different calculation methods
Fetal position	Can affect measurement accuracy
Placental location	May influence growth patterns

Healthcare providers may use additional measurements or formulas based on these factors.

Interpreting results

Fetal weight estimates help monitor growth and identify potential issues:

Normal weight ranges

Early second trimester: 500-1,500 grams
Late second trimester: 1,500-2,500 grams
Term: 2,500-4,500 grams

When to consult a doctor

Estimated weight significantly below or above expected range
Rapid changes in weight over multiple scans
Concerns about fetal growth patterns

Important: This calculator provides an estimate. Always consult with your healthcare provider for personalized advice and monitoring.

Frequently Asked Questions

How accurate are fetal weight estimates?

Fetal weight estimates are generally accurate within 10-15% of the actual birth weight. The Hadlock formula provides a reasonable starting point, though actual birth weight may vary.

When should I get my first ultrasound?

The first prenatal ultrasound is typically recommended between 11-14 weeks of gestation to confirm pregnancy and assess fetal development.

Can I use this calculator for multiple pregnancies?

This calculator uses the standard Hadlock formula which may not be appropriate for multiple pregnancies. Consult your healthcare provider for specialized calculations.

What if my estimated weight is outside the normal range?

If your estimated fetal weight is significantly below or above the expected range, consult your healthcare provider for further evaluation and monitoring.

How often should I get ultrasound measurements?

Ultrasound measurements are typically recommended every 4 weeks during pregnancy, with more frequent scans as needed for high-risk pregnancies.